A permanently modulated index overlay on a waveguide such as a D-fibre will act as a wavelength filter by selectively reflecting a specific wavelength within the waveguide. Gratings scribed on the surface of a D-fibre have also been used as wavelength filters with the guided light impinging on to the grating from within the fibre and the grating sensitive wavelength being reflected.
However the devices produced have the disadvantage that they operate at fixed wavelengths and do not allow for tuning. The techniques employed also relate to optical signals already within a waveguide and do not allow for interfacing with external optical signal sources.
Dynamic diffraction gratings have been generated in a two dimensional slab waveguide of semiconducting material irradiated by two interfering light beams. Bykovskii et al describe such gratings in an article in the Soviet Journal of Quantum Electronics, Vol. 12, No. 4, Apr. 1982, entitled "Investigation of light wave diffraction in a waveguide plane by optically induced dynamic gratings". Techniques for producing dynamic transmission gratings are also described by Sincerbox in published European patent application EP 95563. The devices described both by Bykovskii et al and by Sincerbox are designed to provide angular dispersion of different input wavelengths. As such these devices may be useful, for example, for spectral analysis of a composite optical input.
However, these devices do not address or solve the problem of interfacing a spatially multiplexed optical processor with a one dimensional wavelength multiplexed waveguide.
With wavelength division multiplexed (WDM) systems or coherent systems it is usual to use external modulators each with a separate narrow bandwidth source. It is generally difficult to obtain a large number of sources of slightly different wavelength. WDM implementation may require, for example, grading of a large number of nominally identical lasers or the use of turnable lasers and multiple filters.
The present invention is directed towards providing optical devices which may be used for the desired interfacing function, for example as modulators or filters, and which do not require a large number of separate sources.
For the avoidance of doubt within the context of this specification `optical` includes not only the visible spectrum but also all wavelengths extending into the ultra violet and into the infra red and transmissible by an optical waveguide.
According to the present invention an optical device comprises a one-dimensional optical waveguide including a portion having a refractive index that varies with optical intensity and means for providing at least two coherent waves directed at said portion to form a standing wave therein, thereby to create a modulated refractive index within the waveguide portion.
The term `one-dimensional` is used here to imply that an optical signal will propagate through the waveguide substantially in one-dimension only. This is in particular contrast with slab waveguides, sometimes referred to as `planar` waveguides, where propagation in at least two dimensions is possible.
Preferably, the wavefronts of the waves are arranged to be substantially orthogonal to the longitudinal axis of the variable refractive index portion of the waveguide and the standing wave is created longitudinally in that portion, such that lines of constant refractive index run transversely across the direction of signal propagation along the waveguide.
Conveniently the coherent waves will be plane waves, at least substantially plane where the waves are indicent on the variable refractive index portion of the waveguide.
Preferably the one-dimensional waveguide will comprise an optical fibre. Alternatively, for example, the one-dimensional waveguide may comprise an integrated optical waveguide (eg using InGaAs or the like).
Conveniently, the variable refractive index portion comprises a length of optical fibre with a D-section and an overlay of a suitable non-linear medium whose refractive index varies with optical intensity at one or more wavelengths.
Alternatively, for example, the variable refractive index portion of the waveguide may be provided by doping the waveguide with a medium exhibiting the required non-linear properties. In an optical fibre the non-linear dopant may be incorporated in the fibre core.
Preferably the device further comprises means for adjusting the angular separation between the coherent waves so as to tune the refractive index modulation.
In a preferred embodiment the device includes means for modulating at least one of said coherent waves so as to modulate the standing wave.
Preferably a plurality of different standing waves are established between a common primary beam and a plurality of modulated secondary beams. The primary and secondary beams are conveniently of the same wavelength and may be derived from the same source. Each secondary beam may be differently angularly spaced from the primary beam in order to tune the associated standing wave to reflect and modulate a signal of a particular wavelength within the waveguide.
Also according to the present invention an optical device for modulating wavelength multiplexed optical signals comprises a one-dimensional optical waveguide including a portion having a refractive index that varies with optical intensity, means for providing pairs of coherent waves directed at said portion, each pair having a different angular separation for establishing a plurality of standing waves within the said portion, each standing wave being associated with one of the wavelength multiplexed optical signals and having a wavelength different to each other standing wave, and means for modulating one of each pair of coherent waves to dynamically modulate each standing wave and associated optical signal.
In this preferred embodiment the invention provides an efficient means for simultaneous parallel modulation or filtering of WDM signals in an optical waveguide. This parallel processing facility offers the advantages of more effective use of the bandwidth of optical waveguides and of the speed of optical transmission. For example, using an eight component WDM, the individual electro-optic modulators could potentially be run at the maximum electronic bit rate thereby allowing a full 8-bit word to be transmitted 8 times faster than would be the case for a serial time division multiplex (TDM).
Furthermore, the modulation or filtering for each component wavelength of the WDM signals is achieved by the appropriate angular spacing of the interfering pairs of waves, which are conveniently all of the same wavelength. Consequently, since there is no need to separate each component wavelength of the WDM signals for modulation, the WDM signals may be derived from a single multi-band or spread spectrum source coupling directly into the waveguide. Thus there is no longer a necessity for the complex source grading and filtering mentioned above.
Additionally or alternatively to its use as a modulator or filter, if the wavelengths of the interfering primary and secondary beams are made to differ, then the device may be used to shift the wavelength of signals in the waveguide.
Another aspect of the invention provides a method of modulating an optical signal in a one dimensional optical waveguide comprising causing at least two coherent waves to interfere within a portion of the waveguide that has a refractive index that varies with optical intensity, establishing a standing wave of modulated refractive index within the said portion, and modulating one of the coherent waves to dynamically modulate the standing wave thereby to correspondingly modulate the optical signal.
In a preferred form the invention provides a method of modulating wavelength multiplexed optical signals in a one dimensional optical waveguide comprising providing pairs of coherent waves, each pair having a different angular separation, causing each pair of coherent waves to interfere within a portion of the waveguide that has a refractive index that varies with optical intensity, establishing a plurality of standing waves within the said portion, each standing wave being associated with one of the wavelength multiplexed optical signals and having a wavelength different to each other standing wave, and modulating one of each pair of coherent waves to dynamically modulate each standing wave and associated optical signal.